Curable fluoroelastomer composition

ABSTRACT

Fluoroelastomer compositions comprising fluoroelastomers having copolymerized units of a nitrile-containing cure site monomer are cured with certain carbazates. The carbazate is of the general formula R 1 OC(O)NHNHR 2 , wherein R 1  is 9-fluorenylmethyl, benzyl, aryl, or a heterocycle; R 2  is H, alkyl, aryl, heterocycle, CO 2 R 3  or CH 2 R 4 ; R 3  is alkyl, aryl, benzyl or a heterocycle; and R 4  is a fluoroalkyl group.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/469,856 filed Mar. 31, 2011.

FIELD OF THE INVENTION

This invention relates to curable fluoroelastomer compositions and moreparticularly to fluoroelastomer compositions containing certaincarbazates as curing agents.

BACKGROUND OF THE INVENTION

Fluoroelastomers have achieved outstanding commercial success and areused in a wide variety of applications in which severe environments areencountered, in particular those end uses where exposure to hightemperatures and aggressive chemicals occurs. For example, thesepolymers are often used in seals for aircraft engines, in oil-welldrilling devices, and in sealing elements for industrial equipment thatoperates at high temperatures.

The outstanding properties of fluoroelastomers are largely attributableto the stability and inertness of the copolymerized fluorinated monomerunits that make up the major portion of the polymer backbones in thesecompositions. Such monomers include vinylidene fluoride,hexafluoropropylene, tetrafluoroethylene and perfluoro(alkyl vinyl)ethers. In order to develop elastomeric properties fully,fluoroelastomers are typically crosslinked, i.e. vulcanized. To thisend, a small percentage of cure site monomer is copolymerized with thefluorinated monomer units. Cure site monomers containing at least onenitrile group, for exampleperfluoro-8-cyano-5-methyl-3,6-dioxa-1-octene, are especially preferred.Such compositions are described in U.S. Pat. Nos. 4,281,092; 4,394,489;5,789,489; 5,789,509 and in WO 2011084404.

Bisamidoximes (U.S. Pat. No. 5,668,221) and bisamidrazones (U.S. Pat.No. 5,605,973; 5,637,648) have been used as vulcanizing agents forfluoroelastomers having nitrile group cure sites. These cures may bescorchy, i.e. crosslinking may begin before the final shaping of thecomposition. Also, the curatives require complex, multistep synthesesfrom expensive starting materials.

Other nitrogen containing nucleophilic compounds have been employed tocrosslink fluoroelastomers having nitrile group cure sites (U.S. Pat.No. 6,638,999 B2). Some of these curatives are scorchy while others arevolatile at rubber milling temperatures.

Fluoropolymers having pendant amidrazone or amidoxime groups are alsoknown (U.S. Pat. No. 7,300,985 B2). These polymers require an additionalpolymer modification step in order to form crosslinks.

SUMMARY OF THE INVENTION

The present invention is directed to curable fluoroelastomercompositions which comprise a fluoroelastomer having nitrile group curesites and certain hydrazine derivatives (i.e. carbazates) as curatives.More specifically, the present invention is directed to a curablecomposition comprising:

-   -   A) a fluoroelastomer comprising copolymerized units of a nitrile        group-containing cure site monomer; and    -   B) a carbazate of the general formula R¹OC(O)NHNHR², wherein R¹        is 9-fluorenylmethyl, benzyl, aryl, or a heterocycle; R² is H,        alkyl, aryl, heterocycle, CO₂R³ or CH₂R⁴; R³ is alkyl, aryl,        benzyl or a heterocycle; and R⁴ is a fluoroalkyl group.

Another aspect of the present invention is a cured article made from theabove composition.

DETAILED DESCRIPTION OF THE INVENTION

The fluoroelastomer that may be employed in the composition of theinvention may be partially fluorinated or perfluorinated.Fluoroelastomers preferably contain between 25 and 70 weight percent,based on the total weight of the fluoroelastomer, of copolymerized unitsof a first monomer which may be vinylidene fluoride (VF₂) ortetrafluoroethylene (TFE). The remaining units in the fluoroelastomersare comprised of one or more additional copolymerized monomers,different from said first monomer, selected from the group consisting offluoromonomers, hydrocarbon olefins and mixtures thereof. Fluoromonomersinclude fluorine-containing olefins and fluorine-containing vinylethers.

Fluorine-containing olefins which may be employed to makefluoroelastomers include, but are not limited to vinylidene fluoride(VF₂), hexafluoropropylene (HFP), tetrafluoroethylene (TFE),1,2,3,3,3-pentafluoropropene (1-HPFP), 1,1,3,3,3-pentafluoropropene(2-HPFP), chlorotrifluoroethylene (CTFE) and vinyl fluoride.

Fluorine-containing vinyl ethers that may be employed to makefluoroelastomers include, but are not limited to perfluoro(alkyl vinyl)ethers. Perfluoro(alkyl vinyl) ethers (PAVE) suitable for use asmonomers include those of the formula

CF₂═CFO(R_(f′)O)_(n)(R_(f″)O)_(m)R_(f)   (I)

where R_(f′) and R_(f″) are different linear or branchedperfluoroalkylene groups of 2-6 carbon atoms, m and n are independently0-10, and R_(f) is a perfluoroalkyl group of 1-6 carbon atoms.

A preferred class of perfluoro(alkyl vinyl) ethers includes compositionsof the formula

CF₂═CFO(CF₂CFXO)_(n)R_(f)   (II)

where X is F or CF₃, n is 0-5, and R_(f) is a perfluoroalkyl group of1-6 carbon atoms.

A most preferred class of perfluoro(alkyl vinyl) ethers includes thoseethers wherein n is 0 or 1 and R_(f) contains 1-3 carbon atoms. Examplesof such perfluorinated ethers include perfluoro(methyl vinyl ether)(PMVE), perfluoro(ethyl vinyl ether) (PEVE) and perfluoro(propyl vinylether) (PPVE). Other useful monomers include those of the formula

CF₂═CFO[(CF₂)_(m)CF₂CFZO]_(n)R_(f)   (III)

where R_(f) is a perfluoroalkyl group having 1-6 carbon atoms, m=0 or 1,n=0-5, and Z═F or CF₃. Preferred members of this class are those inwhich R_(f) is C₃F₇, m=0, and n=1.

Additional perfluoro(alkyl vinyl) ether monomers include compounds ofthe formula

CF₂═CFO[(CF₂CF{CF₃}O)_(n)(CF₂CF₂CF₂O)_(m)(CF₂)_(p)]C_(x)F_(2x+1)   (IV)

where m and n independently=0-10, p=0-3, and x=1-5. Preferred members ofthis class include compounds where n=0-1, m=0-1, and x=1.

Other examples of useful perfluoro(alkyl vinyl ethers) include

CF₂═CFOCF₂CF(CF₃)O(CF₂O)_(m)C_(n)F_(2n+1)   (V)

where n=1-5, m=1-3, and where, preferably, n=1.

If copolymerized units of PAVE are present in fluoroelastomers employedin the invention, the PAVE content generally ranges from 25 to 75 weightpercent, based on the total weight of the fluoroelastomer. Ifperfluoro(methyl vinyl ether) is used, then the fluoroelastomerpreferably contains between 30 and 65 wt. % copolymerized PMVE units.

Hydrocarbon olefins useful in the fluoroelastomers employed in theinvention include, but are not limited to ethylene and propylene. Ifcopolymerized units of a hydrocarbon olefin are present in thefluoroelastomers, hydrocarbon olefin content is generally 4 to 30 weightpercent.

The fluoroelastomer further contains copolymerized units of at least onecure site monomer, generally in amounts of from 0.1-5 mole percent.

The range is preferably between 0.3-1.5 mole percent. Although more thanone type of cure site monomer may be present, most commonly one curesite monomer is used and it contains at least one nitrile substituentgroup. Suitable cure site monomers include nitrile-containingfluorinated olefins and nitrile-containing fluorinated vinyl ethers.Useful nitrile-containing cure site monomers include those of theformulas shown below.

CF₂═CF—O(CF₂)_(n)—CN   (VI)

where n=2-12, preferably 2-6;

CF₂═CF—O[CF₂—CFCF₃—O]_(n)—CF₂—CFCF₃—CN   (VII)

where n=0-4, preferably 0-2;

CF₂═CF-[OCF₂CFCF₃]_(x)—O—(CF₂)_(n)—CN   (VIII)

where x=1-2, and n=1-4; and

CF₂═CF—O—(CF₂)_(n)—O—CF(CF₃)CN   (IX)

where n=2-4.

Those of formula (VIII) are preferred. Especially preferred cure sitemonomers are perfluorinated polyethers having a nitrile group and atrifluorovinyl ether group. A most preferred cure site monomer is

CF₂═CFOCF₂CF(CF₃)OCF₂CF₂CN   (X)

i.e. perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene) or 8-CNVE.

A first aspect of this invention is a curable composition comprising A)a fluoroelastomer comprising copolymerized units of a nitrilegroup-containing cure site monomer; and B) a certain carbazate. Thecarbazate may be a mono-, di-, or poly-carbazate. The carbazate is ofthe general formula R¹OC(O)NHNHR², wherein R¹ is 9-fluorenylmethyl,benzyl, aryl, or a heterocycle; R² is H, alkyl, aryl, heterocycle, CO₂R³or CH₂R⁴; R³ is alkyl, aryl, benzyl or a heterocycle; and R⁴ is afluoroalkyl group. Preferably, R² is H or CO₂R³. The alkyl, aryl, benzylor heterocycle groups may contain additional functional groups such as,but not limited to halogen, ether, or amide groups. The fluoroalkylgroup is an alkyl group that has at least 1 of the hydrogen atomsreplaced by fluorine and may optionally contain 1 or more oxygen atomsin the chain. R¹, R², R³, or R⁴ may have 1, 2, or more points ofattachment to carbazate to afford mono-, di-, or poly-carbazate.Compounds that thermally decompose to form one of these carbazates mayalso be employed in the composition of the invention. The carbazate mayalso be a salt, e.g. benzyloxycarbonylhydrazine hydrochloride. Lastly,one or both oxygen atoms may be replaced by sulfur.

Specific examples of these carbazate curatives include, but are notlimited to 9-fluorenylmethyl carbazate; diphenylmethyl carbazate;1-(tert-butoxycarbonyl)-2-phenylhydrazine (i.e. BOC—NHNHPh); phenylcarbazate; and benzyl carbazate. An example of a dicarbazate ishydroquinone dicarbazate. Preferred carbazates include 9-fluorenylmethylcarbazate and benzyl carbazate.

It is theorized that these carbazates act as curing agents by causingthe dimerization of polymer chain bound nitrile groups to form1,2,4-triazole rings, thus crosslinking the fluoroelastomer. Thecarbazates employed in this invention are less volatile than curativessuch as hydrazine or t-butyl carbazate, making the carbazates of theinvention less likely to be fugitive during mixing and shapingprocesses. This is both an industrial hygiene and compound stability(scorch) benefit.

In order to be useful as either the major, or as the only curative forthese fluoroelastomers, the level of carbazate should be about 0.05 to 7parts carbazate per 100 parts fluoroelastomer, preferably about 0.1 to 3parts carbazate per 100 parts fluoroelastomer, most preferably about 0.5to 2 parts carbazate per 100 parts fluoroelastomer. As used herein,“parts” refers to parts by weight, unless otherwise indicated.

An appropriate level of carbazate can be selected by considering cureproperties, for example the time to develop maximum moving die rheometer(MDR) torque and minimum Mooney scorch of the curable compositions. Theoptimum level will depend on the particular combination offluoroelastomer and carbazate.

Optionally, a curative accelerator, e.g. a compound that releasesammonia at curing temperatures, may be used in combination with acarbazate. Examples of compounds that decompose to release ammonia atcuring temperatures include those disclosed in U.S. Pat. No. 6,281,296B1 and U.S. 2011/0009569.

Optionally, another curative commonly employed to crosslinkfluoroelastomers having nitrile-group cure sites may be used in additionto the carbazate. Examples of such other curatives include, but are notlimited to diaminobisphenol AF,2,2-bis(3-amino-4-anilinophenyl)hexafluoropropane, mono- orbis-amidines, mono- or bis-amidrazones, mono- or bis-amidoximes, or anorganic peroxide plus coagent.

Additives, such as carbon black, fluoropolymer micropowders,stabilizers, plasticizers, lubricants, fillers, and processing aidstypically utilized in fluoroelastomer compounding can be incorporatedinto the compositions of the present invention, provided they haveadequate stability for the intended service conditions.

The curable compositions of the invention may be prepared by mixing thefluoroelastomer, carbazate and other components using standard rubbercompounding procedures. For example, the components may be mixed on atwo roll rubber mill, in an internal mixer (e.g. a Banbury® internalmixer), or in an extruder. The curable compositions may then becrosslinked (i.e. cured) by application of heat and/or pressure. Whencompression molding is utilized, a press cure cycle is generallyfollowed by a post cure cycle during which the press cured compositionis heated at elevated temperatures in excess of 300° C. for severalhours.

The curable compositions of the present invention are useful inproduction of gaskets, tubing, and seals. Such cured articles aregenerally produced by molding a compounded formulation of the curablecomposition with various additives under pressure, curing the part, andthen subjecting it to a post cure cycle. The cured compositions haveexcellent thermal stability, steam and chemical resistance. Volume swell(ASTM D1414) after exposure to 225° C. water for at least 168 hours,preferably at least 336 hours is less than 5%. Also compression set,300° C., 70 hours, 15% compression (ASTM D395) is less than 70%. Thecured compositions are particularly useful in applications such as sealsand gaskets for manufacturing semiconductor devices, and in seals forhigh temperature automotive uses.

Other fluoropolymers containing nitrile cure sites, such asfluoroplastics may be substituted for fluoroelastomers in thecompositions of the invention.

The invention is now illustrated by certain embodiments wherein allparts are by weight unless otherwise specified.

EXAMPLES Test Methods Cure Characteristics

Cure characteristics were measured using a Monsanto MDR 2000 instrumentunder the following conditions:

-   -   Moving die frequency: 1.66 Hz    -   Oscillation amplitude: ±0.5 degrees    -   Temperature: 190° C., unless otherwise noted    -   Sample size: Disks having diameter of 1.5 inches (38 mm)    -   Duration of test: 30 minutes

The following cure parameters were recorded:

-   -   M_(H): maximum torque level, in units of dN·m    -   M_(L): minimum torque level, in units of dN·m    -   Tc90: time to 90% of maximum torque, minutes

Test specimens were prepared from elastomer compounded with appropriateadditives, as described in the formulations listed in the Examplesbelow. Compounding was carried out on a rubber mill. The milledcomposition was formed into a sheet and a 10 g sample was die cut into adisk to form the test specimen.

Compression set of O-ring samples was determined in accordance with ASTMD395. Mean values are reported.

Volume swell in water was measured at 225° C. for the time indicated inthe Tables in accordance with ASTM D1414.

The following fluoroelastomer polymer was used in the Examples:

FFKM1—A terpolymer containing 74.7 mole percent units of TFE, 24.6 molepercent units of PMVE and 0.70 mole percent units of 8-CNVE was preparedaccording to the general process described in U.S. Pat. No. 5,789,489.FFKM2—A terpolymer similar to FFKM1, but containing 61.8 mole percentunits of TFE, 37.4 mole percent units of PMVE and 0.8 mole percent unitsof 8-CNVE.

Examples 1-3

Curable compositions of the invention were compounded on a two-rollrubber mill in the proportions shown in Table I. The compoundedcompositions are labeled Example 1 (9-fluorenylmethyl carbazate,available from Glycopep Chemicals, Inc., Chicago, Ill.), Example 2(benzyl carbazate, available from Sigma-Aldrich, St. Louis, Mo.) andExample 3 (BOC—NHNHPh, prepared according to the procedure of K.Kisseljova, et. al. in Organic Letters, 2006, Vol. 8, No. 1, pp. 43-45and Supplemental Information) in Table I. Cure characteristics of thecompounded compositions are also shown in Table I.

O-rings were made by press curing the curable compositions at atemperature of 190° C. for Tc90 plus 5 minutes, followed by a post curein a nitrogen atmosphere at a temperature of 305° C. for 26 hours aftera slow temperature ramp up from room temperature. Compression set valuesand volume swell are reported in Table I.

In order to compare volume swells of the compositions of the inventionwith a prior art composition, o-rings were made from a similar compound,but containing 0.25 phr urea as curative, rather than a carbazate. Afteronly 168 hours of exposure to 225° C. water, the urea cured o-ringsexhibited a 15.7% volume swell.

TABLE I Example 1 Example 2 Example 3 Formulation (phr)¹ FFKM1 100 100 0FFKM2 0 0 100 Carbon Black MT N990 30 30 30 9-fluorenylmethyl carbazate1.92 0 0 Benzyl carbazate 0 1.26 0 BOC—NHNHPh 0 0 1.58 CureCharacteristics M_(L) (dN · m) 2.58 2.49 1.07 M_(H) (dN · m) 11.62 11.026.5 Tc90, minutes 11.5 20.2 22.8 Compression set, 300° C., 31.8 42.968.3 70 hours, 15% compression, % Volume swell, 168 hours, % 0.4 0.81.9² ¹Parts per hundred parts fluoroelastomer ²336 hours

What is claimed is:
 1. A curable composition comprising: A) afluoroelastomer comprising copolymerized units of a nitrilegroup-containing cure site monomer; and B) a carbazate of the generalformula R¹OC(O)NHNHR², wherein R¹ is 9-fluorenylmethyl, benzyl, aryl, ora heterocycle; R² is H, alkyl, aryl, heterocycle, CO₂R³ or CH₂R⁴; R³ isalkyl, aryl, benzyl or a heterocycle; and R⁴ is a fluoroalkyl group. 2.A curable composition of claim 1 wherein R² is H or CO₂R³.
 3. A curablecomposition of claim 1 wherein said carbazate is selected from the groupconsisting of 9-fluorenylmethyl carbazate; diphenylmethyl carbazate;1-(tert-butoxycarbonyl)-2-phenylhydrazine; phenyl carbazate; and benzylcarbazate.
 4. A curable composition of claim 1 wherein said carbazatehas at least one oxygen atom replaced by a sulfur atom.
 5. A curablecomposition of claim 1 further comprising a curative accelerator.
 6. Acured article made from the composition of claim
 1. 7. A cured articleof claim 6 having a volume swell, measured according to ASTM D1414,after exposure to 225° C. water for at least 168 hours of less than 5%and a compression set, 300° C., 70 hours, 15% compression, measuredaccording to ASTM D395 of less than 70%.